A number of lifts have been developed in order to aid handicapped persons, in particular, wheelchair passengers, in boarding and exiting vehicles such as buses, trains, vans, and the like. One type of lift, commonly known as a "platform lift," is disclosed in U.S. Pat. No. 4,058,228 to Hall. The platform lift is designed to be positioned in the stairwell of a vehicle and mounted to the underside of the vehicle frame. The lift is not completely enclosed, and consequently is exposed to outside elements such as moisture, pollutants, dirt, and snow. The lift includes a platform that is slidably mounted beneath the vehicle floor. In an extended position, the platform projects outwardly from the vehicle so that a wheelchair passenger may be carried by the platform. In a stowed position, a part of the platform extends from the vehicle and serves as a step of the passenger access staircase. The platform is attached by a parallelogram linkage assembly to a carriage that is caused to move between extended and retracted positions by a chain drive assembly actuated by a hydraulic or pneumatic actuator. The parallelogram linkage assembly is used to move the extended platform between a vehicle floor-level position and a ground-level position.
Platform lifts of the type described above have been used extensively in buses with very favorable results. Although the lift is exposed to the outside, and thus to environmental elements such as moisture, pollutants, dirt, and snow, this has not been found to be a problem since buses typically are serviced on a relatively frequent basis. Moreover, buses are operated on paved roads and often are used only for relatively short trips. However, in cases in which vehicles have a longer useful life, are serviced less frequently, are operated under conditions that subject the vehicle to impacts from rocks and debris, and/or are used for relatively long trips, it is desirable to provide a lift that is completely enclosed so as to be protected from the environment. For example, a railcar typically has a longer useful life and is serviced less frequently than a bus. Moreover, a railcar is subject to impacts from rocks and debris on railroad tracks and is often used for long trips. In order to protect against damage from outside elements, it is desirable that the lift be completely enclosed when not in use.
Furthermore, a lift of the type having a platform that forms the bottom step of the passenger access stairway is not usable in a vehicle having a fixed passenger access step. For example, some vehicles have a unitary frame structure that includes a ledge in the entryway of the vehicle which serves as a passenger access step. A lift that can be mounted and deployed over the fixed step of the vehicle is desirable.